26 Aug
2020
26 Aug
'20
5:06 a.m.
On 8/24/2020 1:20 PM, Dan Cross wrote:
On Mon, Aug 24, 2020 at 1:08 PM John Cowan
<cowan(a)ccil.org
<mailto:cowan@ccil.org>> wrote:
On Mon, Aug 24, 2020 at 12:00 PM Dan Cross <crossd(a)gmail.com
<mailto:crossd@gmail.com>> wrote:
Stacks may be at the top of the user portion of the address
space; but I'd have to double check the details.
That's always true on the PDP-11 and Vax, no matter what the OS,
because the processor architecture (which has pre-increment and
post-decrement instructions, but not their counterparts) makes
anything but a downward-growing stack unmanageable.
Ah, but if one has a fixed-size stack that cannot be extended, one can
put it anywhere one wants in the virtual address space. E.g., right
after the program text segment or whatever (effectively using the text
as a guard to detect stack overflow). I don't know why one would want
to do that, except that it makes freeing the virtual address space
slightly simpler when the process exits, but the point is that the
Unix choice isn't the only way. That said, stacks and data growing
toward each gives the maximum amount of flexibility.
In OSes without virtual memory like RSX-11[ABC], RT-11, and
mini-Unix/LSX-11, what counts as the top naturally varies.
On TOPS-10, I got into the habit of putting the PDL at the end of the
lowseg. If it ran over, it would die.
ak